This invention pertains to the manufacture of fiberboard ducts for use as insulating conduit for transmitting hot or cold gases, and more particularly to an apparatus and method of forming flat fiberboard into individual hollow duct units.
Linear fiberboard duct is produced from a wide variety of fibrous boards which are formed into hollow duct units having a square or rectangular through-put cross-section. Several duct units are secured end to end to provide a continuous run of duct work such as shown in U.S. Pat. Nos. 3,242,780; 3,420,142; 3,534,646; and 3,605,534. Duct forming apparatus shown in these patents as well as in U.S. Pat. No. 4,070,954 comprise flat fiberboard folded into a rectangular cross section while passing through a plurality of elongated rollers arranged in an off-set parallelogram. The fiberboard is formed into a hollow duct whereby the duct seam is closed and secured with tape. The cross section of the duct can be changed by adjusting the elongated rollers inwardly or outwardly to accommodate larger or smaller cross-sectional duct units. Similarly, U.S. Pat. No. 3,908,526 suggests the formation of fiberboard tubes by coacting pairs of rollers adapted to maintain a hollow tube during the forming process. The prior art processes, however, require exact spacing of the peripheral rollers to maintain the fiberboard in the form of a parallelogram while securing the linear seam of the hollow duct. Thus, prior art processes necessitate maintaining the fiberboard in perfect parallelogram alignment during the formation of each hollow duct unit.
It now has been found that hollow fiberboard duct can be manufactured by folding the fiberboard flat with an upwardly exposed linear seam between the distal ends of the fiberboard, whereby the linear seam engages a depending permanent guide means adapted to maintain the adjacent wall members in butting position while the seam is permanently secured with tape. The centrally disposed permanent guide means avoids adjustments for different cross-sectional ducts and avoids maintaining hollow parallelogram alignment during the forming process. In accordance with the invention, flattened fiberboard is conveyed while the linear abutting edges of the fiberboard duct engages the guide means, which quickly and automatically aligns the adjacently disposed fiberboard wall members into butting position and is maintained in juxtaposition while the linear seam is stapled and sealed. Upon completely securing the linear seam, the flattened duct can be removed and stacked for shipping or expanded for use as hollow duct. The centrally disposed guide means permits accommodations of various size fiberboards without cumbersome adjustments to accommodate variable size cross sections, and further avoids the need for accurate adjustment of pairs of rollers to maintain proper parallelogram alignment. These and other advantages will become more apparent by referring to the drawings and the detailed description of the invention.